Battery not powering 12-volt system

[worksology]

For the past several months we've been without 12-volt power. When we're on shore power, all of our 12-volt appliances & lights still work (converter's still converting), but when that's disconnected NOTHING works at all.

The batteries are getting rather old but I don't believe them to be the problem. Our hitch jack (connected directly to the battery) continues to work while off of shore power after several months, which seems to indicate that not only are the batteries still functional but the charger continues to charge them.

My suspicion is a problem with the battery disconnect or a fuse between the battery & converter (but I'm openly soliciting other opinions/theories). I seem to recall a weird moment when I put it into "store" mode last fall and heard something unusual. I really can't be more specific but that's a suspicion.

So, where should I be working to fix this? Is there a fuse that might have blown between the batteries & the converter or are battery disconnects known to fail? I've checked all the fuses on the distribution panel & they look okay. Again, everything works on shore power.

This is a 2007 27' International.

Many thanks.